Welding of railroad rails is often used to join two rail sections together as a railway is built or repaired. One type of welding is commonly referred to as flash butt welding. During flash butt welding, the two rails ends to be joined are first heated and then forged together, expelling liquid metal and oxides from the weld joint. The forged joint is sheared to remove the flash, which is solidified material that was forced out of the joint during forging.
As the weld surfaces are prepared during certain steps in the flash butt welding process, the rate at which the rail material is manipulated will have a significant impact on weld quality. For example, if the rail segments are moved together too slowly during certain steps, the heated surfaces may oxidize, reducing eventual weld strength. Similarly, if the rail segments are moved together too quickly, the weld will be made at temperature too low to allow the weld to be homogenous from one segment to the other, resulting in occlusions, fractures, and other problems. Thus, it is important to ensure that the rail segment ends are moved at prescribed rates of travel during certain steps of the welding process.
Some present-day welding control systems utilize welding current as a process variable for closed-loop control of the rate of rail displacement during certain steps of the welding process. The welding current between the two rails is measured and if it is found to exceed a target value, the relative velocity of the two rails is deemed to be too high and the rail closing speed (the relative rate at which the two rails are moved toward one another) is reduced. If the welding current is found to be less than the target value, the relative velocity of the two rails is deemed to be too low, and the rail closing speed is increased. The rail closing speed can be controlled by a setting of a hydraulic proportional valve that feeds the closing cylinders on the welding system, and thus the required control output comprises a signal to the hydraulic proportional valve to open or close to some degree. In addition to the closed-loop feedback control based on current, there may be limits placed on the extent to which the control output to the hydraulic proportional valve can change, in order to prevent the system from becoming unstable, i.e., to avoid rate oscillations.
However, in practice, feedback control of the rail movement based on current is not always sufficient to account for and manage the variety of field conditions encountered during welding. For example, the inventors have observed that in some circumstances feedback control based on measured current is not adequate to respond to rapidly changing conditions of the rails, and in some cases the current measured by the welding head does not always adequately represent the current traveling between the rails and contributing to their heating. Thus, an improved control scheme for ensuring consistency of weld quality under varying field conditions is needed.
When considering this background section, the disclosure and claims herein should not be limited by the deficiencies of the prior art. In other words, the solution of those deficiencies, while desirable, is not a critical limitation of any claim except where otherwise expressly noted in that claim. Moreover, while this background section is presented as a convenience to the reader who may not be of skill in this art, it will be appreciated that this section is too brief to attempt to accurately and completely survey the entirety of the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art.